4-Color Custom Set discussion about alliances between enemy color pairs

People have been trying for years to make 4-color card sets work.  But instead of trying to force all four color pips onto a card (like the Nephilim cycle), what if we explore the design space of "split color" mana costs?  

I'll use Ravnica guilds so we can all speak in a common language, but this type of custom set could be made on some other plane than Ravnica, of course.  

For one set, imagine treaties between two Ravnica guilds who both share a common enemy:

• {u/g}&{w/b} = Simic and Orzhov both have the color Red as an enemy.
• {w/b}&{u/r} = Orzhov and Izzet both have the color Green as an enemy.
• {u/r}&{b/g} = Izzet and Golgari both have the color White as an enemy.
• {b/g}&{r/w} = Golgari and Boros both have the color Blue as an enemy.
• {r/w}&{u/g} = Boros and Simic both have the color Black as an enemy.

Or maybe for a different set, imagine alliances between Ravnica guilds who are protecting their friends from a common foe.

• {w/u}&{b/r} = Azorius and Rakdos both have the color Green as a foe.
• {b/r}&{w/g} = Rakdos and Selesnya both have the color Blue as a foe.
• {w/g}&{u/b} = Selesnya and Dimir both have the color Red as a foe.
• {u/b}&{r/g} = Dimir and Gruul both have the color White as a foe.
• {r/g}&{w/u} = Gruul and Azorius both have the color Black as a foe.

There would be NO MONO-COLORED SPELLS.  Each set would ONLY have spells with specific Enemy color pairs (for that first example) or specific Allied color pairs (for the second example).  And then the signpost uncommons would be cards which represent the specific treaties between the guilds which have both split-color guild symbols.  

Thoughts?